1. Field of the Invention
The present invention relates to a liquid crystal display device and a manufacturing method thereof.
2. Description of the Related Art
A liquid crystal display device has been widely used in various fields because of the features such as high display quality, a small thickness, a light weight, and low power consumption, and thus has been widely used for monitors for mobile devices such as those for mobile phones and digital still cameras, monitors for desktop personal computers, monitors for printing and designing, monitors for medical use, and liquid crystal televisions. Along with the expansion in application, higher image quality and higher quality have been required for the liquid crystal display device. In particular, high brightness by high transmittance and low power consumption have been strongly required. Further, along with the popularization of the liquid crystal display device, there has been a strong need for low costs.
In general, a liquid crystal display device displays an image using changes of the optical characteristics of a liquid crystal layer caused by changing the alignment direction of liquid crystal molecules while applying an electric field to the liquid crystal molecules of the liquid crystal layer sandwiched between a pair of substrates. The alignment direction of the liquid crystal molecules when no electric field is applied is regulated by an alignment film obtained by performing a rubbing process on the surface of a polyimide thin film. A conventional active driving-method liquid crystal display device having a switching element such as a thin-film transistor (TFT) for each pixel displays an image in such a manner that an electrode is provided at each of a pair of substrates sandwiching a liquid crystal layer to be set as a so-called vertical electric field in which the direction of an electric field applied to the liquid crystal layer becomes substantially vertical to the surface of the substrate, and the optical rotation of liquid crystal molecules configuring the liquid crystal layer are used. As typical vertical electric-field liquid crystal display devices, a TN (Twisted Nematic) method and a VA (Vertical Alignment) method are known.
In the TN or VA liquid crystal display device, the viewing angle is disadvantageously narrow. Accordingly, an IPS (In-Plane Switching) method and an FFS (Fringe-Field Switching) method are known as display methods to achieve a wide viewing angle. The IPS method and the FFS method are so-called horizontal electric-field display methods in which a comb-like electrode is formed at one of a pair of substrates, and an electric field generated has components substantially parallel to the surface of the substrate. Liquid crystal molecules configuring a liquid crystal layer are allowed to be rotated in a plane substantially parallel to the substrate, and the birefringence of the liquid crystal layer is used to display an image. Due to the in-plane switching of the liquid crystal molecules, the viewing angle is advantageously wider and the load capacity is lower as compared to the conventional TN method. Thus, these methods have been highly expected as new liquid crystal display devices in place of the TN method, and have been rapidly progressed in these days.
A liquid crystal display element controls the alignment state of the liquid crystal molecules in the liquid crystal layer on the basis of the presence or absence of electric fields. Specifically, upper and lower polarizing plates provided outside the liquid crystal layer are placed to be completely orthogonal to each other, and a phase difference is allowed to be generated using the alignment state of intermediate liquid crystal molecules to form bright and dark states. The alignment state when no electric field is applied to liquid crystal can be controlled in such a manner that a polymer thin film called “alignment film” is formed on the surface of a substrate, and the liquid crystal molecules are aligned in the polymer array direction using an intermolecular mutual effect by the van der Waals force of polymer chains and liquid crystal molecules at the interface. The effect is called as providing an alignment restraining force or a liquid crystal alignment capability, or an alignment process.
Polyimide is used for the alignment film of a liquid crystal display in many cases. As a formation method, polyamide acid as the precursor of polyimide is dissolved in various solvents to be applied onto a substrate by spin coating or printing. Then, the substrate is heated at a temperature of 200° C. or higher to remove the solvents, and polyamide acid is put to imidization ring-closing reaction to polyimide. The film thickness at this time is as thin as about 100 nm. The surface of the polyimide thin film is rubbed in a certain direction using a rubbing cloth, so that the polyimide polymer chains on the surface are aligned in the direction to realize a state in which the anisotropy of polymer on the surface is high. However, there are problems such as generation of static electricity and foreign substances caused by rubbing and unevenness of rubbing due to irregularities of the surface of the substrate. Thus, a photo-alignment method that is not required to be contacted with the rubbing cloth and that controls the molecular alignment using polarized light is beginning to be employed.
The photo-alignment methods of the liquid crystal alignment film include a photoisomerization type in which polarized ultraviolet rays such as azo color are irradiated to change the geometric arrangement in molecules, and an optical dimeric type in which molecular frameworks such as cinnamic acid, coumalin, and chalcone are chemically bonded by polarized ultraviolet rays. However, a photodegradative type in which polarized ultraviolet rays are irradiated onto polymer to cut and decompose only the polymer chains aligned in the direction, and the polymer chains in the direction orthogonal to the polarized direction are left is suitable for photo-alignment of polyimide that is reliable and proven as the liquid crystal alignment film.
The principle of such a photo-alignment method is disclosed in, for example, “Nematic Homogeneous Alignment by Photo Depolymerization of Polyimide, by Masaki HASEGAWA and Yoichi TAIRA, proceedings of 20th discussion on liquid crystal, pp. 232 to 233, 1994”. The method was studied using various liquid crystal display methods, among which the IPS method is disclosed in Japanese Patent Application Laid-Open No. 2004-206091 as a liquid crystal display device that reduces generation of display fault due to variation of the initial alignment direction, has stable alignment of liquid crystal, is suitable for mass production, and has high-quality images in which the contrast ratio is increased. Japanese Patent Application Laid-Open No. 2004-206091 shows that alignment control capability is provided by an alignment process in which at least one of secondary processes of heating, irradiation of infrared rays, irradiation of far-infrared rays, irradiation of electron beams, and irradiation of radioactive rays is performed for polyamic acid or polyimide composed of cyclobutanetetracarboxylic dianhydride and/or the derivative thereof and aromatic diamine.
Then, Japanese Patent Application Laid-Open No. 2004-206091 shows that especially, at least one of processes of heating, irradiation of infrared rays, irradiation of far-infrared rays, irradiation of electron beams, and irradiation of radioactive rays is performed together with a polarization irradiation process, so that the alignment process can be further effectively performed. Further, Japanese Patent Application Laid-Open No. 2004-206091 shows that an imidization baking process of an alignment control film is performed together with a polarization irradiation process, so that the alignment process can be further effectively performed. In particular, in the case where at least one of processes of heating, irradiation of infrared rays, irradiation of far-infrared rays, irradiation of electron beams, and irradiation of radioactive rays is performed for the liquid crystal alignment film in addition to the polarization irradiation process, the temperature of the alignment control film is desirably in a range of 100° C. to 400° C. More desirably, the temperature is in a range of 150° C. to 300° C. Japanese Patent Application Laid-Open No. 2004-206091 shows that the processes of heating, irradiation of infrared rays, and irradiation of far-infrared rays are effective because the processes can be performed together with the imidization baking process of the alignment control film.
However, the history of development of the liquid crystal display device using the photo-alignment film is shorter as compared to that using the rubbing alignment film, and there are not sufficient findings in terms of long-time display quality over several years as a practical liquid crystal display device. Specifically, the fact is that a relation between image fault that does not become obvious in the initial stage of manufacturing and a problem unique to the photo-alignment film has been hardly reported.